Masks for filtering dust and other particulates from air are known. In particular, a known mask has a hump-shaped, pliant layer of filter material formed to have an irregularly shaped perimeter to conform relatively closely to a person's face. The mask forms an enclosure about the person's nose and mouth. The known mask includes a short piece of foam-like material in a region where the mask contacts the person's nose. Apparently, the foam-like material functions as a padding. A pair of straps are attached to the mask for holding the mask to the person's head.
The problem with the known mask is that different people have different face configurations and, for that matter, the face configuration of a particular person changes as he breathes, talks, grimaces, or even as he becomes fatigued during working, so that the known mask does not always conform to these various configurations and provide an appropriate seal. More subtly and more importantly, leakage may occur during normal breathing due to the natural spherical concavity formed by the position of the cheekbone, the teeth, and the outer part of the jaw on both sides of the wearer's face. A pliant, muscular tissue covers these concavities to form the cheek. During inhalation, the pressure differential is such that it creates an external force pushing against the cheek area and forcing the cheek tissue inward toward the inside of the mouth. During this process, the cheek tissue may be separated slightly from the perimeter of the known mask. Any slight separation provides an opening so that the small particulates which the mask is intended to filter may be forced into the space enclosed by the mask by the higher pressure which is outside the mask. The problem increases in magnitude anytime the jaw drops thereby increasing the area and depth of the indicated concavity. Although the indicated problem is subtle, the particles which are to be filtered are smaller than the eye can see, and over time this deficiency in the known mask could lead to a serious situation.
Consequently, although the known mask meets the efficiency requirements with respect to the filter material, the mask system which includes attachment to the wearer, often has a much lower efficiency since there either is an inadequate seal between the person's face and the mask at the time the mask is put on, or an inadequate seal develops at various times during the wearing.
In this regard, it has been recognized that a half mask respirator, i.e., one that encloses a person's nose and mouth, can provide much improved protection if anthropological characteristics of people are taken into consideration. See "Fit Factors of an Anthropometrically Designed Three Size Half Mask", Jones, J. A., Brissette, J. E. and Holm, J. M., Journal of the International Society for Respiratory Protection, Volume 5, Issue 4, Oct.-Dec. 1987.
Even considering such fit factors, however, leakage is still a substantial concern. In the authoritative "NIOSH Guide to Industrial Respiratory Protection" Bollinger, N.J., and Schutz, R. H., Department of Health and Human Services, Sept. 1, 1987, it was stated that "no qualitative or quantitative fit tests have been demonstrated to be capable of effectively identifying inadequately fitting respirators." In that regard, the document further indicates, as do publications in the Federal Reqister, that NIOSH, the National Institute for Occupational Safety and Health, is trying to establish significant revisions to 30 C.F.R. part 11, which will become 42 C.F.R. part 84 regarding federal certification for respirators. Apparently, when the new 42 C.F.R. part 84 is approved, it will include the Jones et. al. fit factors as criteria. Nevertheless, industry and government are extremely concerned over respirator leakage and it is clear that subtle changes in design may prove very significant and even life-saving.